Accelerated verification of digital signatures and public keys

ABSTRACT

Accelerated computation of combinations of group operations in a finite field is provided by arranging for at least one of the operands to have a relatively small bit length. In a elliptic curve group, verification that a value representative of a point R corresponds the sum of two other points uG and vG is obtained by deriving integers w,z of reduced bit length and so that v=w/z. The verification equality R=uG+vQ may then be computed as −zR+(uz mod n) G+wQ=O with z and w of reduced bit length. This is beneficial in digital signature verification where increased verification can be attained.

This application claims priority from U.S. Provisional Application No.60/644,034 filed on Jan. 18, 2005, the content of which is herebyincorporated by reference.

The present invention relates to computational techniques used incryptographic algorithms.

BACKGROUND TO THE INVENTION

The security and authenticity of information transferred through datacommunication systems is of paramount importance. Much of theinformation is of a sensitive nature and lack of proper control mayresult in economic and personal loss. Cryptographic systems have beendeveloped to address such concerns.

Public key cryptography permits the secure communication over a datacommunication system without the necessity to transfer identical keys toother parties in the information exchange through independentmechanisms, such as a courier or the like. Public key cryptography isbased upon the generation of a key pair, one of which is private and theother public that are related by a one way mathematical function. Theone way function is such that, in the underlying mathematical structure,the public key is readily computed from the private key but the privatekey cannot feasibly be ascertained from the public key.

One of the more robust one way functions involves exponentiation in afinite field where an integer k is used as a private key and thegenerator of the field α is exponentiated to provide a public keyK=α^(k). Even though α and K are known, the underlying mathematicalstructure of the finite field makes it infeasible to obtain the privatekey k. Public key cryptography may be used between parties to establisha common key by both parties exchanging their public keys andexponentiating the other parties public key with their private key.Public key cryptography may also be used to digitally sign a message toauthenticate the origin of the message. The author of the message signsthe message using his private key and the authenticity of the messagemay then be verified using the corresponding public key.

The security of such systems is dependent to a large part on theunderlying mathematical structure. The most commonly used structure forimplementing discrete logarithm systems is a cyclic subgroup of amultiplicative group of a finite field in which the group operation ismultiplication or cyclic subgroups of elliptic curve groups in which thegroup operation is addition.

An elliptic curve E is a set of points of the form (x, y) where x and yare in a field F, such as the integers modulo a prime p, commonlyreferred to as Fp, and x and y satisfy a non-singular cubic equation,which can take the form y²=x³+ax+b for some a and b in F. The ellipticcurve E also includes a point at infinity, indicated as O. The points ofE may be defined in such a way as to form a group. The point O is theidentity of the group, so that O+P=P+O=P for any point P in E. For eachpoint P, there is another point, which we will write as −P, such thatP+(−P)=P+(−P)=O. For any three points P, Q, R in E, associativity holds,which means that P+(Q+R)=(P+Q)+R. Identity, negation and associativityare the three axiomatic properties defining a group. The elliptic curvegroup has the further property that it is abelian, meaning that P+Q=Q+P.

Scalar multiplication can be defined from addition as follows. For anypoint P and any positive integer d, dP is defined as P+P+ . . . +P,where d occurrences of P occur. Thus 1P=P and 2P=P+P, and 3P=P+P+P, andso on. We also define 0P=O and (−d) P=d (−P).

For simplicity, it is preferable to work with an elliptic curve that iscyclic (defined below) although in practice, sometimes a cyclic subgroupof the elliptic curve is used instead. Being cyclic means that there isa generator G, which is a point in the group such that every other pointP in the group is a multiple of G, that is to say, P=dG, for somepositive integer d. The smallest positive integer n such that nG=O isthe order of G (and of the curve E, when E is cyclic). In cryptographicapplications, the elliptic curves are chosen so that n is prime.

In an elliptic curve cryptosystem, the analogue to exponentiation ispoint multiplication. Thus it is a private key is an integer k, thecorresponding public key is the point kP, where P is a predefined pointon the curve that is part of the system parameters. The seed point Pwill typically be the generator G. The key pair may be used with variouscryptographic algorithms to establish common keys for encryption and toperform digital signatures. Such algorithms frequently require theverification of certain operations by comparing a pair of values as toconfirm a defined relationship, referred to as the verificationequality, between a set of values.

One such algorithm is the Elliptic Curve Digital Signature Algorithm(ECDSA) used to generate digital signatures on messages exchangedbetween entities. Entities using ECDSA have two roles, that of a signerand that of a verifier. A signer selects a long term private key d,which is an integer d between 1 and n−1 inclusive. The integer d must besecret, so it is generally preferable to choose d at random. The signercomputes Q=dG. The point Q is the long term public key of the signer,and is made available to the verifiers. Generally, the verifiers willhave assurance generally by way of a certificate from a CA, that Qcorresponds to the entity who is the signer. Finding the private key dfrom the public key Q is believed to an intractable problem for thechoices of elliptic curves used today.

For any message M, the signer can create a signature, which is a pair ofintegers (r, s) in the case ECDSA. Any verifier can take the message M,the public key Q, and the signature (r, s), and verify whether it wascreated by the corresponding signer. This is because creation of a validsignature (r, s) is believed to possible only by an entity who knows theprivate key d corresponding to the public key Q.

The signing process is as follows. First, the signer chooses someinteger k in the interval [1, n−1] that is to be used as a session, orephemeral, private key. The value k must be secret, so generally it ispreferable to choose k randomly. Then, the signer computes a point R=kGthat has co-ordinates (x, y). Next, the signer converts x to an integerx′ and then computes r=x′ mod n, which is the first coordinate of thesignature. The signer must also compute the integer e=h(M) mod n, whereh is some hash function, generally one of the Secure Hash Algorithms(such as SHA-1 or SHA-256) defined in Federal Information ProcessingStandard (FIPS) 180-2. Finally, the second coordinate s is computed ass=(e+dr)/s mod n. The components (r,s) are used by the signer as thesignature of the message, M, and sent with the message to the intendedrecipient.

The verifying process is as follows. First the verifier computes aninteger e=h(M) mod n from the received message. Then the verifiercomputes integers u and v such that u=e/s mod n and v=r/s mod n. Next,the verifier computes a value corresponding to the point R that isobtained by adding u G+v Q. This has co-ordinates (x, y). Finally theverifier converts the field element x to an integer x′ and checks thatr=x′ mod n. If it does the signature is verified.

From the above, the verification of an ECDSA signature appears to taketwice as long as the creation of an ECDSA signature, because theverification process involves two scalar multiplications, namely uG andvQ, whereas signing involves only one scalar multiplication, namely kG.Elliptic curve scalar multiplications consume most of the time of theseprocesses, so twice as many of them essentially doubles the computationtime. Methods are known for computing uG+vQ that takes less time thancomputing uG and vG separately. Some of these methods are attributed toShamir, some to Solinas, and some to various others. Generally, thesemethods mean that computing uG+vQ can take 1.5 times as long ascomputing kG.

Another commonly used method to accelerate elliptic curve computationsis pre-computing tables of multiples of G. Such pre-computed tables savetime, because the point G is generally a fixed system parameter that isre-used repeatedly. The simplest pre-compute table consists of allmultiples 2{circumflex over (0 )}j G for j from 0 to t, where t is thebit-length of n. With such a pre-computed table, computing an arbitrarymultiple kG can be done with an average of t/2 point additions or less.Roughly, this a threefold improvement over the basic method of computingkG, which clearly demonstrates the benefit of pre-computation. Generallyspeaking, larger pre-computed tables yield better time improvements. Thememory needed to store the pre-computed tables has a significant cost.Therefore, implementers must balance the benefit of faster operationswith the extra cost of larger tables. The exact balance generallydepends of the relative importance of speed versus memory usage, whichcan vary from one implementation to another.

Pre-computation can also be applied to the public key Q. Generally, thepublic key Q tends to vary more often than G: as it is different foreach correspondent, whereas G is always fixed for a given system.Therefore the cost of one-time pre-computation for Q is amortized over asmaller number of repeated run-time computations involving Q.Nevertheless, if Q is to be used more than once, some net savings ontime will be achieved. Public keys that are heavily used include thoseof certification authorities (CA), especially root, trusted or anchor CApublic keys (that are pre-installed into a system). Therefore,pre-computation may be worthwhile for CA elliptic curve public keyswhere, for example, the protocol requires verification of a CA'scertificate. Another difference between pre-computations of Q versus Gis the cost of storing or communicating the pre-computed tables. Eachpublic key Q requires its own pre-computed table. In a system with manydistinct public keys, these costs may accumulate to the point that anybenefit of faster computation is offset by the need to store orcommunicate keys. The net benefit depends on the relative cost of time,memory and bandwidth, which can vary tremendously betweenimplementations and systems. Again, in the case of CA public keys,especially root, trusted or anchor CA keys, these keys tend to be fewerin number than end-entity public keys, so that the cost ofpre-computation will generally be less and amortised over moreoperations.

Tables of multiples of points are not merely useful duringpre-computation. In practice, such tables are commonly generated atrun-time, during an initial phase of each computation. The savingsprovided by these tables is essentially that of avoiding certainrepetitious operations that occur within a single computation. A singlecomputation has less internal repetitions than two distinct computationshave in common, so that saved repetition amount to less thanpre-computation. Nevertheless, it has been found that with a judiciouschoice of table, the time need for a single computation can be reduced.The table takes time to compute, and computation of the table cannot beamortized over multiple computations, so is incurred for everycomputation. Experience has shown that particular tables decrease theamount of time needed because computing the table takes less time thanthe repetition operations that would have otherwise been needed.Usually, there is an optimum size and choice of table. Another cost ofsuch tables is the memory needed to temporarily store the table. Thecost of such memory may affect the optimal choice of table. Windowingmethods are examples of such tables computed on the fly.

Not withstanding all of the above known techniques for efficientimplementation, further efficiency improvements are desirable. Inparticular, the efficiency of verifying of ECDSA signatures isparticularly desirable. Extensive pre-computation allows ECDSAsignatures to be generated very quickly. In fact, ECDSA signaturegeneration is one of the fastest digital signature generation algorithmsknown. On the other hand, ECDSA signature verification is relativelyslower, and there are other signature algorithms have similarverification times to ECDSA. Improvement of ECDSA verification time istherefore important, especially for environments where verification timeis a bottleneck.

In general, there is a need to enhance the efficiency of performing acomputation to verify that a value corresponds to the sum of two of thevalues. It is therefore an object of the present invention to obviate ormitigate the above disadvantages.

SUMMARY OF THE INVENTION

In general terms the present invention provides a method and apparatusfor verifying the equality of a relationship between the sum of scalarmultiples of a pair of points on an elliptic curve and a third point onsaid curve. The method comprises the steps of i) obtaining a pair ofintegers of bit length less than one of said scalars and whose ratiocorresponds to said scalar; ii) substituting said integers for saidscalars in said relationship to obtain an equivalent relationship inwhich at least one of said terms is a scalar multiple of one of saidpoints with reduced bit length, and iii) computing said equivalentrelationship to verify said equality.

The method may be used for verifying that a value representative of apoint R on an elliptic curve corresponds to the sum of two other points,uG and vQ. Integers w and z are determined such that the bit stringsrepresenting w and z are each less than the bit string of the integer u,and such that v=w/z mod n. With such w and z, the equation R=uG+vQ canbe verified as −zR+(zu mod n)G+wQ=O.

Preferably, the bit lengths of w and z are each about half the bitlength of n, which means that both w and z are both no larger than aboutn^(I/2)

The point −zR+(zu mod n)G+wQ can be computed efficiently because z and ware relatively small integers, and various of the methods for computinga sum faster than its parts can be used. The multiple (zu mod n) is fullsize, but within the context of an algorithm such as the ECDSA, thepoint G may be fixed or recurring. In this case the computation can beaccelerated with the use of a stored table for G. Estimates of the timessavings for this approach compared to conventional verification withtables for G are around 40%.

The values w and z may be obtained by using a partial completed extendedEuclidean algorithm computation. Alternatively, a continued fractionsapproach may be utilised to obtain w and z efficiently.

In a further aspect of the invention there is provided a method ofverifying a digital signature of a message performed by a cryptographicoperation in a group of a finite field having elements represented bybit strings of defined maximum bit length. The signature comprises apair of components, one of which is derived from an ephemeral public keyof a signer and the other of which combines the message, the firstcomponent and the ephemeral public key and a long term public key of thesigner. The method comprises the steps of recovering the ephemeralpublic key from the first component, establishing a verificationequality as a combination of group operations on the ephemeral publickey, the long term public key and a generator of the group with at leastone of the group operations involving an operand represented by bitstrings having a reduced bit length less than the defined maximum bitlength, computing the combination and accepting the signature if saidequality holds and rejecting the signature if said equality fails.

Preferably, the group is a elliptic curve group. As a further aspect, amethod of generating a signature of a message by a cryptographicoperation in an elliptic curve group of finite field comprising thesteps of generating a pair of signature components with one of saidcomponents derived from a point representing an ephemeral public key andincluding in said signature an indicator to identify one of a pluralityof possible values of said public key that may be recovered from saidone component.

Embodiments of the invention will now be described by way of exampleonly with reference to the accompanying drawings in which:

FIG. 1 is a schematic representation of a data communication system,

FIG. 2 is a flow chart illustrating the steps in performing a signaturefor an ECDSA signature scheme.

FIG. 3 is a flow chart showing the verification of a ECDSA signature.

FIG. 4 is a flow chart showing the verification of an ECDSA signatureusing a precomputed value.

FIG. 5 is a flow chart showing the verification of an ECDSA signatureusing a table of precomputed values.

FIG. 6 is a flow chart showing the verification of an ECDSA signatureusing a precomputed value provided by the signer

FIG. 7 is a flow chart showing steps taken by a verifier upon failing toverify.

FIG. 8 is a flow chart showing steps taken by a signor to simplifyverification.

FIG. 9 is a flow chart showing an alternative signature protocol tosimplify verification

FIG. 10 is a flow chart showing an alternative technique performed bythe Signor to simply verification.

FIG. 11 is a flow chart showing an alternative verification ECDSA.

FIG. 12 is a flow chart showing point verification.

FIG. 13 is a flow chart showing a modified PVS verification protocol.

FIG. 14 shows a method of recovering a public key from an ECDSAsignature.

The present invention is exemplified by reference to verification ofdigital signatures, in particular those signatures generated usingECDSA. It will be apparent however that the techniques described areapplicable to other algorithms in which verification of a pair of valuesrepresentative of points on an elliptic curve is required to groupsother than elliptic curve groups. Therefore the accompanying descriptionof preferred embodiments is exemplary and not exhaustive.

Referring therefore to FIG. 1, a data communication system 10 includes apair of correspondents 12, 14 interconnected by a transmission line 16.The correspondents 12, 14 each include cryptographic modules 20, 22respectively that are operable to implement one of a number ofcryptographic functions. The modules 20, 22 are each controlled by CPU'sincorporated in the correspondents 12, 14 and interfacing between inputdevices, such as a keyboard 24, a display device 26, such as a screenand a memory 28. Each cryptographic module includes internal processingcapability including a random number generator 30 and an arithmeticprocessor 32 for performing elliptic curve computations such as pointaddition. It will be appreciated that the correspondents 12, 14 may begeneral purpose computers connected in a network or specialised devicessuch as cell phones, pagers, PDA's or the like. The communication link16 may be a land line or wireless or a combination thereof. Similarlythe cryptographic modules 20,22 may be implemented as separate modulesor incorporated as an application within the CPU.

In the present example, the correspondent 12 prepares a message M whichit wishes to sign and send to the correspondent 14 using an ellipticcurve cryptosystem embodied within the modules 20, 22. The parameters ofthe system are known to each party including the field over which thecurve is defined (in the present example Fp where p is a prime), theunderlying curve, E, the generator point G that generates the elementsthat form the group in which crypto operations are performed andtherefore defines the order, n, of the group, and a secure hash functionH, generally one of the Secure Hash Algorithms (such as SHA-1 orSHA-256) defined in Federal Information Processing Standard (FIPS)180-2. Each element is represented as a bit string having a maximum bitlength sufficient to represent each element in the group.

The steps taken to sign the message are shown in FIG. 2. Initiallytherefore the correspondent generates an integer k by the random numbergenerator 30 and utilises the arithmetic unit 32 to compute a point R=kGthat has co-ordinates (x, y). The correspondent 12 converts theco-ordinate x to an integer x′ and computes r=x′ mod n, which is thefirst component of the signature. The correspondent 12 also computes theinteger e=H(M) mod n, where H is the secure hash function. Finally, thesecond component s is computed as s=(e+dr)/k mod n.

In addition to the components r and s, the signature includesinformation i to permit the co-ordinates representing the point R to berecovered from the component r. This information may be embedded in themessage M, or forwarded as a separate component with r and s and will beused by the verifier to compute the value R If the elliptic curve isdefined over a field F of prime order p, and the elliptic curve E iscyclic or prime order n, then i can generally be taken as y mod 2, i.e.,a zero or one. The indication i is required during recovery R, where theverifier sets x=r. It is very likely that x=r because n and p areextremely close for typical implementations. Given x, there are exactlytwo values y such that (x, y) is on the curve, and these two values yand y′ have different values mod 2. Thus i is just a single bit whosevalue indicates which of the y's is to be used, and adds relativelylittle cost to the signature.

Once the message is signed it is forwarded together with the componentsr,s, and i across the link 16 to the recipient correspondent 14. Toverify the signature the steps set out in FIG. 3 are performed. Firstthe correspondent 14 computes an integer e=H(M) mod n. Then thecorrespondent utilises the arithmetic unit 32 to compute a pair ofintegers u and v such that u=e/s mod n and v=r/s mod n.

The correspondent 14 also computes a pair of integers w and z using aniterative algorithm such that the maximum bit lengths of w and z areeach less than the maximum bit length of the elements of the group, andsuch that v=w/z mod n. The bit lengths of w and z are preferably aboutone half the bit length of the elements. Such w and z can be foundconveniently with the extended Euclidean algorithm and stopping at anappropriate point, typically half-way where w and v are half the bitlength of the elements. Such an algorithm is exemplified, for example asAlgorithm 3.74 in Guide to Elliptic Curve Cryptography by Henkerson,Menezes and Vanstone published by Springer under ISBN 0-387-95273, whichrepresents a quantity k as k=k₁+k₂ λ mod n, where the bit lengths of k₁and k₂ are about half the length of n. This equation can be re-writtenas λ=(k−k₁)/k₂ mod n. By setting k=1 and λ=v, then the above referencedAlgorithm 3.74 can be used to obtain n established for the system, k setto 1 and the value for v used as the variable input. The output obtainedk₁ k₂ can then be used to compute w=1−k₁ and k₂ used as w=1−k₁ and z=k₂.

Thus, the arithmetric unit 32 is used to implement the followingpseudo-code to obtain the values of w and z.

Let r0=n and t0=0.

Let r1=v and t1=1.

For i>1, determine ri, ti as follows:

Use the division algorithm to write r_(i−1)=qi r_(i-2)+ri, which definesri.

Let ti=t_(i−1)+qi t_(i−2).

Stop as soon as ri<sqrt(n)=nˆ(½), or some other desired size.

Set w=ri and z=ti. Note that ri=ti v mod n, so w=z v mod n, so v=w/z modn, and both w and z have about half the bit length of n, as desired.

The correspondent 14 also recovers a value corresponding to the point Rutilising the information i. In its simplest form this is obtained bysubstituting the value of r received in the curve and determining whichof the two possible values of y correspond to the sign indicated by thebit i.

With the value of R recovered, the verification of the ECDSA, namelythat R=uG+vQ, may proceed with a revised verification by confirming thatthe verification equality −zR+(zu mod n)G+wQ=O. The verificationequality −zR+(zu mod n)G+wQ involves a combination of group operationson each of the ephemeral public key R, generator G and long term publickey Q and can be computed efficiently because z and w are relativelysmall integers. As will be described below, various of the methods forcomputing a sum faster than its parts can be used. The multiple (zu modn) is full size, but within the context of a system such as the ECDSA inwhich the points have varying longevity, the point G may be consideredfixed or recurring. In this case the computation can be accelerated witha precomputed table for G, which may be stored in the memory 28 andaccessed by arithmetric unit 32 as needed. The representations of thepoints −zR and wQ which cannot effectively be precomputed have smallerbit lengths and therefore less time consuming computation. Assuming thecomputation returns a value O, the signature is assumed to be verified.

A number of different known techniques may be utilised to compute therequired relationship, each of which may be implemented using thearithmetic processor 32. Each offers different advantages, either inspeed or computing resources, and so the technique or combination oftechniques will depend to a certain extent on the environment in whichthe communication system is operating. For the sake of comparison, itwill be assumed that u and v are integers of bit length t. Computing uGand vQ separately requires about 3t/2 point operations, assuming nopre-computation, for a total of about 3t point operations. ComputinguG+vQ, which is the verification normally used for ECDSA, require tdoublings, some of which can be simultaneous. Each of u and v areexpected to have about t/2 bits set to one in their binaryrepresentation. In basic binary scalar multiplication, each bit of onerequires another addition. (In more advanced scalar multiplication,signed binary expansion are used, and the average number of additions ist/3.) The total number of point operations is therefore t+(2(t/2))=2t onaverage as simultaneous doubling has saved t doublings.)

The revised verification instead uses a computation of a combination ofthe form aG+wQ+zR, where a is an integer of bit length t representativeof the value zu mod n and w and z are integers of bit length about(t/2). Organising the verification computation in this way permits anumber of efficient techniques to be used to reduce the number of pointoperations. An efficient way to compute this is to use a simultaneousdoubling and add algorithm. For example, if the relationship 15G+20Q+13Ris to be computed it can be done in stages as 2Q; G+2Q; G+2Q+R;2G+4Q+2R; 3G+4Q+2R; 3G+5Q+2R; 3G+5Q+3R; 6G+10Q+6R; 7G+10Q+6R;14G+20Q+12R; 15G+20Q+13R, for a total of 12 point additions, which isfewer than the method of generic scalar multiplication for each termseparately. The main way that this method uses less operations is whenit does simultaneous doubling, in steps as going from G+2Q+R to2G+4Q+2R. In computing each term separately three operations would beused corresponding to this one operation. In fact, three simultaneousdoubling were used, each saving two operations, so simultaneous doublingaccount precisely for all the savings. The number of doublings tocompute the combination is governed by the length of the highestmultiple, so it is t. The number of additions for a is (t/2), onaverage, and for Q and R it is (t/4) each on average. The total, onaverage, is t+(t/2)+(t/4)+(t/4)=2t. The algorithm is further exemplifiedas Algorithm 3.48 of the Guide to Elliptic Curve Cryptography detailedabove.

Although there does not appear to be any savings over the previousmethod, which also took 2t point operations, advantage can be taken ofthe fact that in practice, for ECDSA, the generator G is constant. Thisallows the point J=2ˆm G to be computed in advance, and stored in memory28 for future use. If m is chosen to be approximately t/2, then a ≡a′+a″2ˆm, where a′ and a″ are integers of bit length about (t/2).Accordingly, aG+wQ+zR can be written as a′G+a″J+wQ+zR. In this form, allthe scalar multiples have bit length (t/2). The total number ofdoublings is thus (t/2). Each of the four terms contributes on average(t/4) additions. The total number of point operations, on average, isthe t/2+4(t/4)=3t/2.

Accordingly, to verify the signature r,s, as shown schematically in FIG.4, the recipient computes w, and z as described above, determines thevalue of a′ and a″ and performs a double and add computation to obtainthe value of the verification representation. If this corresponds to thegroup identity, the verification is confirmed.

With the conventional verification equation approach of computing uG+vQ,the multiple v will generally be full length t, so the pre-computedmultiple J of G will not help reduce the number of simultaneousdoublings.

Therefore, by pre-computing and storing the single point J, verifyingusing the relationship −zR+(zu mod n)G+wQ=O allows an ECDSA signature tobe verified in 25% less time. In other words, 33% more signatures can beverified in a given amount of time using the embodiment described above.

Alternatively, many implementations have sufficient memory 32 topre-compute and store essentially all power of two multiples of G,essentially making it unnecessary to apply doubling operations to G. Insuch situations uG+vQ can be computed with t doublings of Q and (t/2)additions for each of G and Q. The total is still 2t operations.However, as shown in FIG. 5, the value of aG can be retrieved from theprecomputed table stored in memory 32 so that computing aG+wQ+zR, can beattained with (t/2) doublings for the wQ and zR, no doublings for G, t/2additions for G, and t/4 additions for each of Q and R. The total is3t/2 operations. The savings are the same as described with FIG. 4, whenonly one multiple of G was pre-computed and stored.

When signed binary expansions are used, then computing uG+vQ (withoutany pre-computation) requires about t doublings and (t/3) additions foreach of G and Q, for a total of (10/6)t operations, on average. Whensigned binary expansions are used to find a′G+a″J+wQ+zR, about t/2doublings are needed, and (t/6) additions for each of G, J, Q and R, fora total of (7/6)t operations, on average. The time to verify using theverification representation described above is 70% compared to without,or 30% less. This allows about 42% more signatures to verified in agiven amount of time. The advantage of verifying using the revisedverification representation is increased when combined with a moreadvanced technique of scalar multiplication referred to as signed binaryexpansions. This technique is very commonly used today in elliptic curvecryptography, so today's existing implementations stand to benefit fromadoption of the verification representations.

Accordingly, it will be seen that by reorganizing the verificationequation so that signature variables have a reduced bit length, thespeed of verification may be increased significantly.

In the above embodiments, the recipient performs computations on thecomponents r,s. To further accelerate signature verification as shown inFIG. 6, a signer may provide a pre-computed multiple of the public key Qto the verifier. The verifier can use the pre-computed multiple tofurther accelerate verification with Q. In this embodiment the verifierdetermines an equivalent equation to the ECDSA verification equation inthe form aR+bG+cQ=O, where a is approximately n^(1/3) and represents −z,b is approximately n and represents −zu mod n, and c is approximatelyn^(2/3) and represents w. This can be done using the extended Euclideanalgorithm as described above and stopping when the bit length of w istwice that of z. Again, therefore, the signor signs the message M andgenerates the signature components r,s. It also includes the identifieri in the message forwarded to the verifier. The signor pre-computes amultiple βQ where the scalar multiple β is a power of two nearest ton^(1/3) and forwards it with the signature.

Upon receipt, the verifier computes w and z. The verifier thendetermines c=c′+c″β and b=b′+b″β+b″Δβˆ2. In addition, since G is a fixedparameter, the verifier has pre-computed multiples of G of the form βGand βˆ2G. If n is approximately 2ˆt, then the verifier needs just t/3simultaneous doubles to compute aR+bG+cQ. The verification can proceedon the basis aR+(b′+b″β+b′″βˆ2)G+(c′+c″β)Q=O. The precomputed values forG and Q can then be used and the verification performed. The verifierwill need 2t/3 point additions, assuming that signed NAF is used torepresent a, b and c. The total number of point operations is thus t,which represents a further significant savings compared to 3t/2 with thepresent invention and without the pre-computed multiple of Q such asdescribed in FIG. 4 and compared to 2t using the conventionalrepresentations and without any pre-computed multiple of Q.

Given a pre-computed multiple of both Q and G, then uG+vQ can becomputed with (t/2)+4(t/4)=3t/2 point operations using conventionalrepresentations. When pre-computed multiples of Q are feasible, then thesigning equation, in the form above, again provide a significantbenefit. The analyses above would be slightly modified when signedbinary expansions are used.

With yet other known advanced techniques for computing linearcombinations of points, some of which are discussed below, the use ofthe relationship allows signature verification to take up to 40% lesstime.

When implementing scalar multiplication and combinations, it is commonto build a table at run-time of certain multiples. These tables allowsigned bits in the representation of scalar multiple to be processed ingroups, usually called windows. The table costs time and memory tobuild, but then accelerates the rest of the computation. Normally, thesize of the table and associated window are optimized for overallperformance, which usually means to minimize the time taken, except onsome hardware implementation where memory is more critical. A fulldescription and implementing algorithms for such techniques is to befound in Guide to Elliptic Curve Cryptography, referenced above at pages98 et seq.

Such run-time tables, or windowing techniques, for scalar multiplicationtechniques can be combined with the revised verification equation in theembodiments described above. When using such tables, the savings areapproximately the same as outlined above. The reason the savings aresimilar is the following simple fact. Tables reduce the number of adds,by pre-computing certain patterns of additions that are likely occurrepeatedly, whereas the use of the revised verification relationshipreduces the number of doubles by providing for more simultaneousdoubling. In fact, when using tables, the number of adds is reduced, sothe further reduction of the doubles provided by using the revisedverification relationship has even more impact.

By way of an example, a common approach to tables, is to use a signedNAF window of size 5. Building a table for such a NAF requires 11 adds.In the example above where the signer sends a pre-computed multiple uQof Q, the verifier can build tables for R, Q and uQ, at a cost of 33adds. It is presumed that verifier already has the necessary tablesbuilt for G. Using the pre-computed doubles, the verifier only needs t/6simultaneous additions for verification. These savings improve as thekey size increases. For the largest key size in use today, the savingsare in the order of 40%. Such tables do of course require the necessarymemory 32 and so selection of the appropriate techniques is governed bythe hardware available.

Similarly, computation techniques known as joint sparse forms could beused for computational efficiency.

As described above, the integers w, z were found using the extendedEuclidean algorithm. Alternative iterative algorithms may be used,including the continued fractions approach. In the continued fractionsapproach, which is essentially equivalent to the extended Euclideanalgorithm, one finds a partial convergent γ/δ to the fraction v/n, suchthat δ is approximately n^(1/2). A property of the partial convergent isthat |γ/δ−v/n|<1/δˆ2. Multiplying this inequality by δn gives|γn−vδ|<nδ, which is approximately n^(1/2). Now set z=δ and w=γn−vδ. Itis easy to check that v=w/z mod n, and note that w and z have thedesired size.

As noted above, a conventional ECDSA signature, does not include thepoint R but instead, it includes an integer x′ obtained from r=x mod n,where R=(x, y). The verifier therefore needs to recover R.

The method to. recover R discussed above is to supplement the signature(r, s) with additional information i. This information can be embeddedin the message, for example. The verifier can use r and i to compute R.When p>n, there is a negligible chance that x′>n and consequently r=x−n.If however such a case does occur, the verification attempt will fail.Such a negligible failure rate for valid signatures can be accepted, ordealt with in the following manner.

As shown in FIG. 7, upon failure of the verification, at the verifier'sexpense the verifier can try x=r+n, and repeat the verification foranother value of R, which will succeed in this particular case.Continued failure to verify will lead to rejection of the signature.Alternatively, as shown in FIG. 8 the signer can detect when x>n, whichshould happen negligibly often, and when this happens generate adifferent k and R. In either of the approaches, the problem arises sorarely that there is negligible impact on performance.

Other techniques for determining R can be utilized. In non-cycliccurves, there is a cofactor h, which is usually 2 or 4 in practice. Thiscan lead to multiple possible values of x. The probability that r=x isapproximately 1/h. In other situations, we will generally have r=x−n (ifh=2), or more generally r=x−mn where (m is between 0 and h−1). Because pis approximately hn, then except in a negligible portion of cases therewill be h possible values of x that are associated with r. To makerecovery of x, and hence R easier, the signer can compute m and send itto the verifier within the message or as a further signature component.Alternatively, the verifier can make an educated guess for m. This canbe illustrated in the case of h=2.

Corresponding to r is a correct x and a false value x_(f). The falsevalue x_(f) has an approximately ½ chance of not corresponding to avalue of the x-coordinate on E, and a further 1/h chance of notcorresponding to a multiple of G. If one of the two values for x isinvalid, either by not being on E or if it is not having order n, bothof which can be efficiently checked, then that value can be eliminated.Thus at least ¾ of the time, the verifier will easily find the correctx. The remaining ¼ of the time at maximum, the verifier will not knowwhich of the two x-values to try. If the verifier can guess one of the xvalues to verify, half the time, this guess will be right and thesignature will verify, and the other half of the time, the firstsignature attempt will fail and the verifier must try the other x value.Therefore the probability that the verifier must verify two signaturesis ⅛. Despite this probability of two verifications, the averageverification is still improved. This can still provide the verifier timesavings on average. If an accelerated verification takes 70% as long asa normal verify, but 12.5% of the verifies require twice as long as anaccelerated verify, then the average time is 79% of a normal verify.Furthermore, as outlined further below, the signer can assist byproviding m for the verifier or by doing the extra work of trying bothvalues to ensure that only one m is valid.

A similar method may be utilized with a cofactor h=4. In fact, a highervalue of h reduces the probability of each of the potential x valuesfrom being valid. There are more potential x values, but the analysisshows a similar benefit to the verifier. There are three false values ofx, and each has a probability of 1/8 of appearing valid with a fastcheck. The chance that no false values appear to be a valid x with afast check is thus (7/8)³ which is about 77%. Most of the remaining 23%of the time, just one of the false x values will appear valid andpotentially require a full signature verification.

The inclusion of i (and of m if necessary) is quite similar to replacingr by a compressed version of R consisting of the x coordinate and thefirst hit of the y coordinate. This alternative, of sending a compressedvalue of R instead of r, has the advantage of being somewhat simpler andnot even a negligible chance of false recovery. Accordingly, as shown inFIG. 9, the signature is computed to provide a pair of components, r′,sand forwarded with the message M to the recipient 14. The component r′is composed of the x co-ordinate of the point R and the first bit of they co-ordinate. The component s is computed as before.

To verify the signature, the recipient 14 recovers the point R directlyfrom the component r′ and uses the verification equality equation−zR+(zu mod n)G+wQ=O to confirm it corresponds to the group identity.The transmission of the modified co-ordinate r′ simplifies theverification but does increase the bandwidth required.

In some situations, no channel may be available for the signer to sendextra bits. For example, existing standards may strictly limit both thesignature format and the message format leaving no room for the senderto provide additional information. Signers and verifiers cannevertheless coordinate their implementations so that R is recoverablefrom r. This can be arranged by the signer, as shown in FIG. 10, byensuring that the value of x conforms to prearranged criteria. In thenotation above, the signer will compute R=kG=(x, y) as normal, and thenin notation above compute i=y mod 2. If i=1, the signer will change k to−k mod n, so that R changes to −R=(x, −y) and i changes to 0. When theverifier receives the signature, the verifier presumes that i=0, andthus recovers the signature. The value of i is thus conveyed implicitlyas the value 0, and the signer has almost negligible cost for arrangingthis. Similarly, in non-cyclic elliptic curves, the signer may try totransmit m implicitly, which to some extent has already been described.In the case of h=2, recall that the ¼ of the time, the verifier may needto verify two signatures. Instead of the verifier doing this extra work,the signer can detect this ¼ case, and try another value for k and Rinstead, repeating the process until one is found that conforms to thecriteria. The verifier can determine which value of x to use withoutverifying two signatures.

As an alternative to modifying R as described above, and to maintainstrict conformity to the ECDSA standard, the value of s may be modifiedafter computation rather than R. In this case, the signer notes that thevalue of R does not conform to the prearranged criteria and proceeds togenerate r and s as usual. After s is computed, the value is changed to(−s) to ensure the verification will be attained with the presumption ofthe prearranged value of y.

When a signer chooses a signature (r, s) such that R is implicitlyrecovered, an ordinary verifier will accept the signature as usual. Suchsignatures are perfectly valid. In other words, the revised verificationis perfectly compatible with existing implementations of ECDSAverification. An accelerated verifier expecting an implicitly efficientsignature but receiving a normally generated signature, will need to trytwo different values of i. If accelerated verification takes 60% of thetime of a normal verify, then in a cyclic curve (cofactor h=1), theaverage time to needed verify a normal signature is 50% (60%)+50%(120%)=90% of a normal verify. This because 50% of the time a normalsignature will have i=0, requiring just one implicitly acceleratedverify, and the other 50% of the time, two accelerated verifies areneeded. Thus an implicitly accelerated verify will still be faster thana normal verifier, even when the signatures are not implicitlyaccelerated.

Conventional signatures may also be modified, either by the signer or bya third party, to permit fast verification. In this case the signatureis forwarded by a requestor to a third party who verifies the signatureusing the verification equality. In so doing the value of R isrecovered. The signature is modified to include the indicator I andreturned to the requestor. The modified signature may then be used insubsequent exchanges with recipients expecting fast verify signatures.

The above techniques recover R to permit a revised verification usingthe relationship −zR+(zu mod n)G+wQ=O. However, where the ECDSA is beingverified, the integers w and z may be used without recovery of R asshown in FIG. 11. It is possible to compute the x coordinate of zR andthe x′ coordinate of the point wQ+(zu mod n)G, and then check theequality of these two x-coordinates. Only the x-coordinate of the pointzR can be computed, as it is not possible to compute the y-coordinate zRdirectly without knowing the y-coordinate of R. However, there areseveral known methods to compute the x-coordinate of zR from thex-coordinate of R without needing the y coordinate of R. Such techniquesinclude Montgomery's method set out on page 102 of the Guide to EllipticCurve Cryptography identified above. It is then sufficient to check thex-coordinates of zR and wQ+(zu mod n)G, because equality of thex-coordinates means that wQ+(zu mod n)G equal zR or −zR, which means w/zQ+uG equals R or −R, which means uG+vQ has the same x-coordinate as R.This is the condition for successful ECDSA validation. One recovers thex-coordinate of R from the signature component r using the methodsdiscussed above. The advantage of this approach is that it does notrequire extra work to recover the y-coordinate. A disadvantage, comparedto the previous methods above, is that the zR has to be computedseparately from wQ+(zu mod n) G meaning that some of the savings of thejoint sum are not achieved.

The above examples have verified a signature between a pair ofcorrespondents 12, 14. The technique may also be used to verify anelliptic curve key pair (d, Q) as shown in FIG. 12. To verify the keypair means to check that Q=dG. This may be important when the key pairis provided by a third party under secure conditions to ensure notampering has occurred. If t is the bit length of d, then computing dGwith the binary method take (3t/2) operations on average. In the presentembodiment, one of the correspondents, 12, 14 generates a random integerd and obtains a pair of integers w, z such that d=w/z mod n. Typicallythe integers w, z are each of half the of length d. Then thecorrespondent computes zQ−wG, and checks that the result is the groupidentify 0. Computing zQ−wG takes t operations on average so a saving of50% is obtained. This has the most advantage in environments wherestoring a pre-computed multiple of G is too expensive. As an alternativewhere limited memory is available, given a pre-computed multiple H=uG,then dG can be computed as d′ G+d″ H. where d=d″+d″u mod n, with roughlythe same cost as above.

Another application is implicit certificate verification. Implicitcertificates are pairs (P, I), where P is an elliptic curve point and Iis some identity string. An entity Bob obtains an implicit certificatefrom a CA by sending a request value R which is an elliptic curve pointto the CA. The CA returns the implicit certificate (P, I) and inaddition a private key reconstruction data value s. Bob can use s tocalculate his private key. More generally, any entity can use s toverify that the implicit certificate correctly corresponds to Bob'srequest value R and the CA public key C. This is done by checking theverification equality H(P, I)R+sG=H(P,I) P+C, where H is a hashfunction. This equation is equivalent to eQ+sG=C, where e=H(P, I) andQ=R−P. The form of this equation is highly similar to the form of thestandard ECDSA verification equation. Consequently, the techniquesdiscussed above may be used to provide a means to accelerateverification of this equation. This is done optimally by determiningrelatively smaller values w and z such that e=w/z mod n, thenmultiplying the equation through by z to give: wQ+(sz mod n)G−zC=O.Again, the multiple of G is this equation is full size, but generallymultiples of G can be pre-computed, so this does not represent aproblem.

Another variant that takes advantage of this technique is to shorten allthree multiples in the ECDSA signing equation. Theoretically, eachmultiple can be shortened to a length which is ⅔ the length of n (wheren is the order of G). One way to achieve this shortening is by solvingthe short vector lattice problem in 3 dimensions. Algorithms exist forsolving such problems. Shortening all three multiples is most usefulwhen no pre-computed multiples of G can be stored, which makes it moreefficient to reduce the length of the multiple of G as much as possible.Such techniques are described more fully in Henri Cohen, “A Course inComputational Algebraic Number Theory”, Springer, ISBN 0-387-55640-0.Sections 2.6 and 2.6 describe the LLL algorithm, its application tofinding short vectors in lattices, and also mentions Vallee's specialalgorithm for 3 dimensional lattices.

Another application of this technique is the application to a modifiedversion of the Pintsov-Vanstone Signature scheme (PVS) with partialmessage recovery. A PVS signature is of a triple (r, s, t). Verificationof a signature and message recovery from a signature under public Q,with base generator G, is done as follows. The verifier computese=H(r∥t), where H is a hash function. The verifier then computesR=sG+eQ. Next, the verifier derives a symmetric encryption key K from R.With this, the verifier decrypts r using K to obtain a recovered messagepart u. The recovered message is some combination of t and u. Thesignature is valid only if u contains some redundancy, which is checkedby verifying that u conforms to some predetermined format. The PVSscheme is part of draft standards IEEE P1363a and ANSI X9.92.

In a modified variant of PVS, verification time can be decreased byutilizing integers w and z. The modified variant of PVS is shown in FIG.13 and proceeds as follows. After computing e as usual, the verifierthen finds w and z are length half that of n such that e=w/z mod n,where n is the order of the point G. The verifier then computes R=(wsmod n)G+z Q, and proceeds as before, so deriving a key from R and thendecrypting r with the key, and then verifying that the decryption hasthe correct form. This form of verification is more efficient becausethe multiple of Q is smaller.

A method to further accelerate signature verification of digitalsignature, in elliptic curve groups and similar groups is illustrated asfollows. The verification of an ECDSA signature is essentiallyequivalent to confirmation that a linear combination, such as aR+bQ+cG,of three elliptic curve points, equals the point of infinity. One way toverify this condition is to compute the point aR+bQ+cG and then check ifthe result is the point O at infinity, which is the identity element ofthe group as described above. This verification can sometimes be donemore quickly than by directly computing the entire sum. For example, ifa=b=c, then aR+b Q+cG=O if and only if the points R, Q and G arecollinear. Checking if points are collinear is considerably faster thanadding to elliptic curve points. Collinearity can be checked with justtwo field multiplication, by the equation(x_(R)−x_(G))(y_(Q)−y_(G))−(x_(Q)−x_(G))(y_(R)−y_(G))=0. Adding pointsrequires at least two field multiplication, a field squaring and a fieldinversion, which is generally equivalent to about 8 fieldmultiplication. When a=b=c, verification is thus possible in about 18%of the time taken by adding the points. As such, this technique may beused as a preliminary step of the verification process where thelikelihood of these conditions existing is present.

Similarly, when b=c=0, so that one wishes to verify that aR=O, inprinciple one does not need to compute aR in its entirety. Instead onecould evaluate the a^(th) division polynomial at the point R. Thedivision polynomial essentially corresponds to a recursive formula forthe denominators of coordinates the point aR, when expressed as rationalfunctions of the coordinates of the point R. It is known that aR=O ifand only if the denominator is zero. Furthermore, when b=c=0 and theelliptic curve group is cyclic of prime order n, it is known that aR=Oonly if a=0 mod n or if R=O. This verification is comparablyinstantaneous, in that zero elliptic curve point operations are needed.When the cofactor is small, such as h=2 or h=4, point operations canreplaced by a few very fast field operations. Thus special cases ofverification that a sum points is zero can be done very quickly.

Recursive formula exist, similar to the recursive formulae for divisionpolynomials, for the denominators of sums like aR+bQ+cG, and these canbe compute more quickly than the computing the full value of the pointaR+bQ+cG. Knowledge of the group order n can further improveverification time.

Yet another application of this technique is to provide efficientrecovery of the public key Q from only the ECDSA digital signature asshown in FIG. 14. Suppose that one correspondent 12 signs a message Mwith signature (r, s) and wishes to send the signed message to thecorrespondent 14. Normally correspondent 14 will send M and (r, s) tothe correspondent, and will also often send the public key Q. Ifcorrespondent 12 did not send her public key, then normallycorrespondent 14 will look up her public key up in some database, whichcould stored locally or remotely via some network. To avoid this, itwould be beneficial to be able to recover the public key Q from thesignature.

Given an ordinary ECDSA signature (r, s), one can recover severalcandidate points Q that could potentially be the public key. The firststep is recover the point R. Several methods have already been describedfor finding R in the context of accelerated verification, such as: byinclusion of extra information with the signature; by inclusion of extrainformation in the message signed; by extra work on the signer's part toensure one valid R can correspond to r; and by extra work on theverifier's part of trying a multiplicity of different R valuescorresponding to r. Once R is recovered by one of these methods, thenthe public key Q can be recovered as follows. The standard ECDSAverification equation is R=(e/s)G+(r/s)Q, where e=H(M) is the hash ofthe message. Given R and this equation, solving for Q is done byQ=(s/r)R−(e/r) G.

However, since with a significant probability a pair (r, s) will yieldsome valid public key, the correspondent 14 needs a way to check that Qis correspondent's 12 public key. Correspondent 12 can make available tocorrespondent 14 the signature, such as another ECDSA signature (r′,s′), from a CA on correspondent 14 public key. Correspondent 12 can sendthe CA signature, (r′, s′), to correspondent 14, or correspondent 14 canlook it up in some database. The CA's signature will be oncorrespondent's 12 name and her public key Q. Correspondent 14 will usethe CA's certificate to verify the message which corresponds to thepublic key Q. If the signature verifies then the correspondent 14 hasrecovered the correct value for the public key Q. Omitting the publickey from the certificate can save on bandwidth and storage and theverification process described above yields reduced verification times.

Correspondent 14 could also verify that Q is correspondent's 12 publickey by checking Q against some more compact value derived from Q, suchas the half of the bits of Q. The compact version of Q could then storedor communicated instead of Q, again savings on storage and bandwidth.

H will also be appreciated that each of the values used in theverification equality are public values. Accordingly, where limitedcomputing power is available at the verifier it is possible for thesigner to compute the values of w and z and forward them with R as partof the message. The recipient then does not need to recover R or computew and z but can perform the verification with the information available.The verification is accelerated but the bandwidth increased.

Although the descriptions above were for elliptic curve groups, many ofthe methods described in the present invention applies more generally toany group used in cryptography, and furthermore to any other applicationthat uses exponentiation of group elements. For example, the presentinvention may be used when the group is a genus 2 hyperelliptic curve,which have recently been proposed as an alternative to elliptic curvegroups. The above techniques may also be used to accelerate theverification of the Digital Signature Algorithm (DSA), which is ananalogue of the ECDSA. Like ECDSA, a DSA signature consists of a pair ofintegers (r, s), and r is obtained from an element R of the DSA group.The DSA group is defined to be a subgroup of the multiplicative group offinite field. Unlike ECDSA, however, recovery of R from r is not easy toachieve, even with the help of a few additional bits. Therefore, thepresent technique applies most easily to DSA if the value is R sent withas part of the signed message, or as additional part of the signature,or as a replacement for the value r. Typically, the integer r isrepresented with 20 bytes, but the value R is represented with 128bytes. As a result, the combined signature and message length is about108 bytes longer. This could be a small price to pay to accelerateverification by 33%, however.

In the DSA setup, p is a large prime, and q is smaller prime and q is adivisor of (p−1). An integer g is chosen such that gˆq=1 mod p, and1<g<p. (Note that q and g correspond to n and G, respectively, fromECDSA.)

The private key of the signer is some integer x and the public key isY=gˆx mod p.

The signer generates a signature in the form (R,s) instead of the usual(r, s). Here, R=gˆk mod p, whereas, r=R mod q. In both cases,s=kˆ(−1)(h(M)+x r) mod q, where x is the private key of the signer, M isthe message being signed, and h is the hash function being used todigest the message (as in ECDSA).

In normal DSA, the verifier verifies signature (r, s) by computingu=h(M)/s mod q and v=r/s mod q, much like the u and v in ECDSAembodiments, and then checks that r=(gˆu Yˆv mod p) mod q.

In this embodiment, the verifier finds w and z of bit length about halfthat of q, so that each of w and z is approximately sqrt(q), such thatv=w/z mod q. This is done by the same method as in ECDSA embodimentabove, with n replaced by q. The verifier then computes:

Rˆz gˆ(zu mod q) Yˆw mod p.

If this quantity equals 1, then verifier accepts the signature,otherwise the signature is rejected.

The verifier computes this quantity using the square-and-multiplyalgorithm, or some variants thereof, and exploits simultaneous squaring,which is analogous to simultaneous doubling in ECDSA. Many of themethods of ECDSA fast verify may be used for DSA fast verify. Apre-computed multiple of the g, say j, may be used, so that thecomputation looks like: Rˆz gˆS jˆt YˆW mod p where each of z, s, t andw has bit length about half that of q. If pre-computed powers of thepublic Y are made available, then the lengths of the exponents can befurther reduced, thereby further reducing the number of doublings,making the verification yet faster.

1. A method of verifying the equality of a relationship between the sumof scalar multiples of a pair of points on an elliptic curve and a thirdpoint on said curve comprising the steps of i) obtaining a pair ofintegers of bit length less than one of said scalars and whose ratiocorresponds to said scalar, ii) substituting said integers for saidscalars in said relationship to obtain an equivalent relationship inwhich at least one of said terms is a scalar multiple of one of saidpoints with reduced bit length, and iii) computing said equivalentrelationship to verify said equality.
 2. A method according to claim 1wherein a pair of said terms have reduced bit length.
 3. A methodaccording to claim 2 wherein the other of said integers modifies saidthird point to provide a scalar multiple of reduced bit length
 4. Amethod according to claim 3 wherein a further term of said equivalentrelationship is separated in to a pair of points, each represented by abit string of reduced length, one of which has a fixed value and theother of which is variable.
 5. A method according to claim 4 whereinsaid one of said points of fixed value is pre-computed and stored forrepeated use.
 6. A method according to claim 3 wherein said integershave equal bit length.
 7. A method according to claim 3 wherein saidintegers have different bit lengths.
 8. A method according to claim 3wherein said equality is verified by equating the sum of said points toa group identity and performing simultaneous addition on at least a pairof said points.
 9. A method according to claim 8 wherein computation ofthe addition of at least said pair of points is performed using asimultaneous double and add algorithm.
 10. A method according to claim 3wherein said points are used in a system in which one of said points isfixed and the other of said points vary, said variable points beingassociated with said integers of reduced bit length.
 11. A methodaccording to claim 10 wherein at least a portion of the scalar multipleof said fixed point is precomputed.
 12. A method according to claim 11wherein said scalar multiple of said fixed point is split in to a pairof points and one of said points is precomputed.
 13. A method accordingto claim 12 wherein scalar multiples of each of said points are summedand the result compared to the group identity for verification of theequality.
 14. A method according to claim 1 wherein said integers areobtained using an iterative algorithm that is interrupted when integersof required bit length are obtained.
 15. A method according to claim 14wherein said algorithm is extended Euclidean algorithm.
 16. A method ofverifying a digital signature of a message performed by a cryptographicoperation in a group of a finite field having elements represented bybit strings of defined maximum bit length, said signature comprising apair of components, one of which is derived from an ephemeral public keyof a signer and the other of which combines said message, said firstcomponent and said ephemeral public key and a long term public key ofsaid signer, said method comprising the steps of recovering saidephemeral public key from said first component, establishing averification equality as a combination of group operations on saidephemeral public key, said long term public key and a generator of saidgroup with at least one of said group operations involving an operandrepresented by bit strings having a reduced bit length less than saiddefined maximum bit length, computing said combination and acceptingsaid signature if said equality holds and rejecting said signature ifsaid equality fails.
 17. A method according to claim 16 wherein saidgroup operation of reduced bit length is performed on said ephemeralpublic key.
 18. a method according to claim 17 wherein group operationson said ephemeral public key and said long term public key each involvean operand of reduced bit length.
 19. A method according to claim 16wherein said group is an elliptic curve group and said combination ofgroup operations is a sum of scalar multiples of points on said curve.20. A method according to claim 19 wherein said verification equality isof the form −zR=(zu mod n) G+wQ where R is the ephemeral public key; Gis the generator of the group; Q is the long term public key; n is theorder of the group respective; z, w are integers having bit lengths t<n,and u is an integer derived from said signature components O is thegroup identity.
 21. A method according to claim 20 wherein said firstsignature components, r=x′ is an integer derived from the ephemeralpublic key kG where k is an ephemeral private key and said secondsignature component s=k−1 (H(M)=dr)/ where H(M) is a secure has of thesaid message, and d is the long term private key of said signer, andwhere said integer u corresponds to H(M)/_(s) mod n.
 22. A methodaccording to claim 21 wherein said signature components satisfy therelationship uG+vQ=R where v−r/s mod n and v=w/z mod n.
 23. A methodaccording to claim 22 wherein said integers w,z are obtained byapplication of an iterative algorithm that is interrupted when bitlengths of w and z are at a predetermined value.
 24. A method accordingto claim 23 wherein said predetermined bit lengths are equal.
 25. Amethod according to claim 20 wherein precomputed values of G areutilised during computation of said linear combination.
 26. A methodaccording to claim 25 wherein simultaneous double and add operations arepreformed during computation of said linear combination.
 27. A methodaccording to claim 25 wherein said point derived from said generator asplit into a pair of points and are of said points is precomputed.
 28. Amethod according to claim 25 wherein a table of values of 2^(i)G iscomputed and values from said table used in said computation.
 29. Amethod according to claim 19 wherein the co-linearity of said points istested to determine the validity of said quality.
 30. A method accordingto claim 19 wherein said signature includes an indicator to identify onof a plurality of possible values of said ephemeral public key.
 31. Amethod of generating a signature of a message by a cryptographicoperation in an elliptic curve group of finite field comprising thesteps of generating a pair of signature components with one of saidcomponents derived from a point representing an ephemeral public key andincluding in said signature an indicator to identify one of a pluralityof possible values of said public key that may be recovered from saidone component.
 32. A method according to claim 31 wherein said indicatoris a bit of a coordinate of said point representing said ephemeralpublic key.
 33. A digital signature of a message obtained usingcryptographic operations in an elliptic curve cryptosystem, saidsignature including a first component derived from a pointrepresentative of an ephemeral public key of the signer and an indicatorto identify are of a plurality of values of said public key recoverablefrom said first component.
 34. A digital signature according to claim 33wherein said indicator is a bit of a coordinate of said point.
 35. Amethod of generating a digital signature of a message obtained usingcryptographic operations in an elliptic curve cryptosystem comprisingthe steps of generating a point representative of an ephemeral publickey, determining if coordinates of said point meet prearranged criteriaand inhibiting use or said point if said criteria are not met.
 36. Amethod according to claim 35 wherein said point is modified if saidprearranged criteria is not met.
 37. A method according to claim 36wherein said criteria is the value of a first bit of are saidcoordinates of said point.
 38. A method according to claim 35 whereinsaid point is rejected if said criteria are not met and a further pointgenerated.